Many oil shales and other formations contain a great deal of usable hydrocarbons and precursors to usable hydrocarbons, but there are many difficulties in removing the hydrocarbons. Oil shale contains a hydrocarbon precursor known as kerogen, which is a complex organic material that can mature into usable hydrocarbons when exposed to high temperature and/or pressure. Generally, the natural processes that convert kerogen to hydrocarbons occur slowly over many years.
Considering the example of oil shale, it does not have a definite geological definition, and oil shales vary considerably in their content, composition, age, type of kerogen, and other properties. In fact, many will refer to certain formations as oil shale even though the rock may not technically be shale. Regardless, there is a general consensus on various terms used to describe shale characteristics, and people generally characterize oil shale as mature when a large portion of the kerogen has converted to hydrocarbons, and conversely as immature when little of the kerogen has converted to hydrocarbons. One measure of formation maturity is vitrinite reflectance. For example, an immature shale would have mostly kerogen, little or no liquid or gaseous hydrocarbons, and a vitrinite reflectance of less than about 0.5%; a shale of medium maturity would have appreciable amounts of kerogen, liquid hydrocarbons, and gaseous hydrocarbons and a vitrinite reflectance of between about 0.5% and 1.1%; and a shale of high maturity would have very little kerogen, relatively little liquid hydrocarbons, a large quantity of gaseous hydrocarbons, and a vitrinite reflectance of greater than about 1.1%.
In nature, when oil shales mature and the kerogen converts to usable hydrocarbons, the pore pressures throughout the shale generally increase, and the usable hydrocarbons can generate enough pressure to create numerous fractures within the shale matrix material, allowing a portion the hydrocarbons to migrate to places of lower pressure. Other natural sources for fracturing include tectonic fractures and structural variation fracturing. In some circumstances, the network of fractures allows the hydrocarbons to migrate to a large common area of good porosity that may serve as an easily produced oil reservoir. Back at the original oil shale, however, once the pressure has been relieved by the portion of escaping hydrocarbons, the network of fractures will close, trapping the remaining hydrocarbons in the oil shale.
In less mature oil shales, the pressure may not yet have increased enough to create a network of fractures. In these cases, if one wants to gain access to the kerogen to generate hydrocarbons, he must artificially create fractures in the oil shale by applying external pressure—that is, the fracturing force must be applied external to the oil shale to be produced (such as by fluid fracturing or explosion). Even with relatively mature oil shales, technology has generally fractured the rock from the outside-in. Once the formation is fractured, technology is applied to produce any existing usable hydrocarbons through the induced fractures and/or to convert the kerogen to hydrocarbons at a rate much faster than would happen naturally.
Such conversion of kerogen is called pyrolysis and/or retorting. In one known method, kerogen-containing shale can be mined, crushed, and heated to high temperatures to convert the kerogen to liquid hydrocarbons. In other methods, kerogen is converted to hydrocarbons in situ (in place) by heat from combustion, electric or other heaters, and other heating methods and the resulting hydrocarbons are extracted.
As discussed, if the formation is not porous enough to allow the hydrocarbons to travel to a producing well, the shale is fractured via hydro-fracturing techniques or explosives to produce relatively large fissures which serve as conduits to carry hydrocarbons. Once the shale is fractured, it may also be heated in situ to release gases and oils through the large fissures.
It is also known in the art to perform in situ combustion to enhance recovery of oil and gas from a formation. Such methods may introduce an oxidant such as air or other oxygen-containing fluid into the formation, which then reacts exothermically with constituents of the hydrocarbon and/or rock system.
The exothermic reaction in the formation generates heat and other by-products. In conventional in situ combustion, the heat liberated is transferred to the hydrocarbon in the formation causing vaporization and mobilization of a lighter portion of the hydrocarbons which, when mixed with the remaining hydrocarbons results in a lowering of the hydrocarbon viscosity. This in turn increases the mobility of the hydrocarbons and facilitates its movement toward a production well via permeable routes or outside-in fractures. Additionally, as combustion occurs within the formation, the gases produced tend to increase the pressure in a region of the formation near the combustion front. The resulting differential pressure in the formation assists in moving remaining hydrocarbon toward a region of lower pressure, such as a production well.
An example can be found in U.S. Pat. No. 4,042,027, titled “Recovery of petroleum from viscous asphaltic petroleum containing formations including tar sand deposits,” which teaches use of in situ combustion to accomplish thermal cracking and in situ hydrogenation to reduce the viscosity of the reservoir fluid and so to produce hydrocarbons in tar sands.
However, the prior applications of in situ combustion have relied on combustion to reduce viscosity of the reservoir hydrocarbons, such as by hydrocarbon distillation and subsequent dilution in a frontal advance displacement through a permeable matrix. Such prior applications of in situ combustion generally involve relatively permeable reservoirs such as tar sands, and do not involve less permeable reservoirs, such as shale. If the matrix is not naturally permeable, the prior applications will rely on induced fracturing to create permeability. Further, such methods do not rely on heat conduction and/or convection through the shale to create fluid expansion and subsequent mobility/matrix permeability improvement. Moreover, prior methods of in situ combustion have led to a production of primarily gaseous hydrocarbons, leaving valuable liquid hydrocarbons in the formation.
What is needed is a method for heat conduction and/or convection through low-permeable formations, such as shale, to create fluid expansion and subsequent mobility/matrix permeability improvement. What is needed is a method for producing hydrocarbons in less permeable reservoirs without relying primarily on artificially induced fracturing to create permeability. What is needed is a method for producing hydrocarbons that beneficially rejuvenates, from within the internal matrix porosity, an existing matrix of fractures to produce hydrocarbons, especially liquid hydrocarbons.